In order to achieve a relatively high efficiency of a generator, in particular of a generator of a wind turbine, the air gap between the rotor elements, e.g. permanent magnets, and the stator elements, e.g. a laminate stack with windings, of the generator should be relatively small, generally in the range of only a few millimetres even for large generators like direct drive or directly driven generators.
To avoid mechanical damage to the rotor and stator elements it is thereby necessary to ensure that the outer surfaces of the rotor elements and the outer surfaces of the stator elements which are arranged oppositely to each other do not come into direct contact with each other during operation of the generator. Consequently the relatively small air gap required for efficiency reasons must be maintained at a nominal value within narrow tolerances.
In a modern wind turbine a direct drive generator can have a diameter of several meters. Maintaining the air gap in the range of a few millimetres between the rotor elements and the stator elements of a generator of such dimensions requires a comparatively rigid and therefore massive and heavy support structure, in particular of the stator arrangement. This in turn tends to increase the loads on the bearings of the generator and necessitates large, massive and expensive bearings.
Up to now there is a trade-off between the advantages of a small air gap between the rotor and stator elements and the disadvantages of a comparatively rigid and massive support structure. As a result, in particular direct drive generators for wind turbines often operate with larger air gaps between the rotor and stator elements as required for an optimum efficiency, to eliminate the risk of a mechanical damage due to a contact between the rotor and stator elements during operation of the generator.
Additionally large generators are often not so rigid that an air gap adjusted during manufacturing of the generator is maintained after transportation and installation of the generator. As a consequence large generators are often fitted with means for adjusting the air gap after transportation or installation. Such adjustment means increase the complexity of the generator and thus the costs of the generator.
In US 2006/0097582 A1 a generator is described, wherein the rotor of the generator is rotatably journalled to the stator of the generator via bearings arranged in or adjacent to the air gap between the rotor and the stator. This solution reduces the requirements for a rigid support structure in particular of the stator, but it requires large and expensive bearings. Furthermore this solution has a rather long tolerance chain determining the final dimension of the air gap and correspondingly determining the minimum air gap required to eliminate the risk of a mechanical damage due to a contact of the rotor and stator elements during operation of the generator.